1. Field of the Invention
The present invention pertains to fiber-reinforced and non-reinforced thermoplastic honeycomb structural materials. More particularly, the subject invention pertains to a continuous process for the preparation of thermoplastic honeycomb having complex three dimensional shapes without resort to current complex and time-consuming machining operations.
2. Description of the Related Art
The use of honeycomb materials in the aerospace and transportation industries is increasing. While metal honeycomb is satisfactory for many purposes, the need for weight reduction coupled with strength and high modulus has led to the use of honeycomb formed from fiber-reinforcement in a polymer matrix. The polymer matrix may be of a thermosetting or thermoplastic matrix. Honeycombs of engineering thermoplastics not containing fibrous reinforcement have also been used.
A particular advantage of polymer matrix honeycomb is the absorption spectrum for electromagnetic radiation in the microwave region. The transparency to such radiation may be enhanced by selection of the particular polymer, i.e., the polyimides; and by selection of the fiber reinforcement, i.e., high strength polyethylene or quartz. Inclusion of metal flakes or powders, iron oxides or metal coated fibers may also modify electromagnetic behavior. Finally, adjustment of the honeycomb core size to cause destructive interference of particular wavelengths will cause rapid attenuation of such radiation.
In order to utilize honeycomb materials to their best advantage in radar-sensitive applications or other more prosaic applications, it is frequently necessary to produce a part which is contoured as opposed to a flat honeycomb "sandwich." An example of such an application is an airfoil leading edge.
In the past, polymer matrix honeycomb has been produced laboriously by hand layup followed by batch consolidation into a honeycomb "block." This honeycomb block could then be sliced into flat sections which were bonded adhesively between facing sheets to form lightweight, rigid panels. However, the slitting process is expensive, time consuming and entails a relatively large amount of waste. For example, with honeycomb of thin cell wall thickness, the honeycomb must first be filled with polyethylene glycol wax or a similar substance to support the cell walls during the cutting operation. This substance must then be completely removed in order that the core may be adhesively bonded to the facing sheets. When saws are used to cut through the honeycomb, the material in the kerf area is wasted, thus limiting the amount of honeycomb material which can be prepared from a given size block.
The same procedures must be followed when contoured shapes must be produced except that the amount of waste is generally considerably higher, and the cutting operation must be performed by expensive three dimensional computer controlled cutting equipment. The time consuming, waste intensive, and expensive machining operations has limited the use of honeycomb material in contoured applications. Parts which are produced for such applications are extremely expensive.
U.S. Pat. No. 3,356,555 to Jackson discloses a batch method of preparing honeycomb core wherein a squirrel cage roller containing hexagonal bars operates in conjunction with a plurality of hexagonal major bars on a fixed bed to provide a corrugated web by the action of compressing a heated thermoplastic web between the advancing squirrel cage bars and the fixed bed bars. The corrugated webs thus produced are then assembled into a honeycomb structure by placing a first corrugated web on the major bars of a bed followed by placing minor bars in the nodes above the web. A second corrugated sheet is then located with its nodes adjacent to the first web's antinodes. An additional layer of minor bars is placed on top and the procedure repeated until the desired honeycomb thickness is achieved. The assembly is then placed between caul plates and platens and heated to form a finished honeycomb product.
In GB-A-2 188 866, a batchwise method of preparing thermoplastic honeycomb is disclosed wherein shaped formers as disclosed by Jackson in U.S. Pat. No. 3,356,555 are disposed between adjacent corrugated thermoplastic sheets, the assembly pressurized, and heated to cause the thermoplastic to fuse. Following the preparation of one multiple cell honeycomb segment by such batchwise operation, the section may be advanced to allow a further section to be laid up and fused. The process described requires pressurizing and heating the entire newly formed honeycomb structure.
The related art processes produce honeycomb cores in batchwise fashion requiring lengthy heating and cooling cycles to process the honeycomb, or by expansion methods which are directed to but limited honeycomb sizes and to honeycombs not having optimal properties. The processes of Jackson '555 and Great Britain '866 require the use of numerous layers of expensive metal formers. For example, a four foot length of 0.125 cell width honeycomb having but a height of four cells would require in excess of 1400 metal formers. Furthermore, such batchwise processes are time consuming, and require pressurizing an entire honeycomb structure to enable thermoplastic fusion and adherence.
Furthermore, because the abutting node and antinode surfaces of web material is double the thickness of the non-abutting surface, this thickness disparity creates a displacement error in assembling honeycombs using metal formers. Because the error is magnified by each successive cell layer, it is virtually impossible to keep such cores aligned during layup and during cure under pressure. If the formers are made unsymmetrical, i.e. a flattened hexagon, then layup is facilitated, but distortion still occurs during consolidation as the double thickness abutting layers are compressed during fusion to approximately 80 percent of their initial thickness. Honeycomb prepared by all these processes must be cut and/or machined to produce honeycomb parts of other then planar shape.
In the parlance of one skilled in the art of honeycomb production and usage, the thickness of the honeycomb is considered as the dimension parallel to the corrugations, the length is the direction along the same surface as the corrugations but at a right angle thereto, and the width is the direction across the cells perpendicular to the plane of the corrugations (thickness and length directions).